计算机网络复习笔记第五部分--无线网络协议（英文版本）

Wireless Network

Adhoc自组织网络 piconet微微网 :

• Bluetooth
• a master and up to seven slave devices, located within 10m from the master, with data rate 400 to 700kbps

Wireless LAN:

• Wifi (IEEE 802.11)
• up to 100m coverage, with data rate 11 to 54Mbps
• analogous相似的 function to wired LAN

Cellular radio network 蜂窝无线网络：

• mobile phone network, with area of coverage divided into a large number of small cells
• (e.g.) GSM

Bluetooth

General characters

• operates at ISM (industrial, scientific, medical) band.
  – 2.4GHz, no license required for use
• 79 separate carrier frequencies within the ISM band
  – hopping from one frequency to another in a pseudo伪 random order, at a rate of 1600 hops per second (slot time of 625µs per hop) 伪随机码控制载波跳变。接收端发送端都有相同的跳频发生器
  – minimising interference干扰 from other users
  – transmissions within 1,3 or 5 consecutive连续的 slots，可以在后面的传输图里看出来
• modulation bit rate of 1MHz, using the FSK
• system timing provided by the master’s clock, and shared by all slaves within a piconet
  – unique hopping sequence determined by the master
• named after Bluetooth, 10C king of Denmark/Norway

Topology

SS: standby slaves device 备用从设备。无法参与到微微网中来
AS: active slaves，最多7个可以在一个微微网中同时操作
PS: parked slaves 停驻的从属服务器。最多可以255个，不是活跃状态。master知道它的存在，可以被重新激活reactivated。
中间的那个AS可以是两边任何一个微微网的成员，但是not both。由于不同的随即跳变序列，多个微微网可以同时操作。
每个设备可以在任何时间转换角色变成master

Packet transmission

所有的从master到slave的传输都是双数的frequency slot，从slave到master则是单数的。

Frame format （contains packet type）

• access code (68bits): 一定会有信道(哪个微微网)，设备(呼唤的slave)和下一个要接入的neighbor
  preamble plus synchronization for
  – channel access code: identification of the piconet (derived派生的 from the master address)
  – device access code: paging the slave by the master
  – inquiry查询 access code: searching address for the neighbor
• packet header (54):
  – active member address: identification of one of the seven active slaves (0 is reserved保留 for the broadcast packet)
  – packet type (后面会提): data structure
  – flow control: 1(stop transmission) or 0 (resume继续) for the ACL mode
  – acknowledgement type: 1(ACK) or 0(NAK)
  – error check on the header field
• data field (0-2744):
  – additional header, data
  – error check on the data field (CRC)

Packet Type

• Synchronous connection oriented link 面向同步链接的链路 (SCO)
  for speech and audio applications
• asynchronous connectionless link 异步无连接链路 (ACL)
  for general data applications
• poll packet 轮询数据包
  used by the master to poll slaves
• hopping synchronization packet 跳频同步包
  for clock and hopping sequence synchronization 时钟和跳频序列的同步
• acknowledgement packet 确认包
  for error control scheme 差错控制方案

Link manager protocol 链路管理器协议

• establishes links between devices.
  transitions between standby, active, and park states
  

• responsible for security, synchronization, power control, etc.

Service discovery protocol 服务发现协议

• determines available services within the piconet
• operates in a client-server mode
  – searching function: to find a particular service
  – browsing浏览 function: to discover available services

Application

communication between devices in a small area, that does not require high bandwidth.
– PC input/output devices: mouse, keyboard, printer
– game consoles 游戏机: Sony PlayStation3, Nintendo Wii

simply setup and advertises all services it provides, thus making services more accessible 简化设置并且公布了它所提供的所有服务，使得服务更容易访问

Wifi

General characters

• operates at 2.4GHz (ISM band)
  – the ISM band is license free but congested (e.g., microwave oven, bluetooth)
  – bit rate of 11Mbps (802.11b) and 54Mbps (802.11g)
  – coverage of up to 100 meters
• consists of an access point (AP) and clients
  – shares the same protocol above the MAC layer so that WiFi can work together with Ethernet
  – cannot detect collision, instead relies on an < ACK > packet (i.e., retransmission if no acknowledgement is returned within a predefined time).
• WiFi for ‘wireless fidelity’ 无线保真度、

Benefits:

• access network from convenient locations (wherever you want to)
• wireless LAN requires a single access point only, while traditional wired network needs physical cables which sometimes could be not possible
• cheaper than wired network, no need of cables and labor 劳动力

Disadvantages:

• poor quality antenna of the network card needs to be compensated by a powerful transceiver (网卡的劣质天线需要用一个强大的收发器来补偿)
• typical range with standard equipment is up to 100m. may not cover a large structure
• wireless signals will be influenced by interference and complex propagation effect. So important network resources like server, are rarely connected wirelessly
• slow (up to100Mbps) than wired networks (at least 100Mbps)
• security problems: hackers only need a good quality antenna, rather than physical limitation of tapping (窃听) into a real wire

Data flow Topology

CSMA/CA ( Carrier Sense Multiple Access / collision avoidance)

• a node listens the activity of the channel
• previous transmission is completed, all nodes wait for random number of slots (contention window)
  – less slots, more chance to collision; more slots, longer delay
  – basic idea is to prevent collision at the moment they are most likely to occur 在敌人最容易出现的地方驻守
• a node starts transmission
• a node waits for an < ACK > signal (packet received successfully)

Problems in the wireless channel:

• unable to use collision detection (CD): wireless LANs cannot see any signal but its own
• a hidden node: a node A, in the range of B but not C, cannot know whether C is transmitting to B

CSMA/CA with RTS/CTS:

• exchange of
  – a request to send (RTS) packet by the sender
  – a clear to send (CTS) by the intended预定的 receiver
• alerting all nodes within the range to keep quiet for the duration of the main packet

Frame format

• frame control:
  protocol version
  – type: data (10), management (00), control (01).
  – subtype: data (0000), RTS (1011), CTS (1100), ACK (1001).
  – more fragments片段: set to 1, if another fragment of the frame is to follow.接下来是另一片段
  – more data: set to 1, if there is more data to send after this frame.
  – retry: set to 1, if the frame is a retransmission of the earlier frame.
  – power management: set to 1 after the successful transmission, if the node goes into power save mode.
• duration: indicates the frame length in microseconds.
• address 1−4: source and destination addresses, etc.

WiFi protected access (WPA) Wifi 保护访问

• RC4 stream cipher RC4流密码
• 128bit key/48 bit initialization vector
• enterprise mode企业模式: different keys to each other
  pre-shared key mode: same pass to all users
• temporal key integrity protocol (TKIP) 临时密钥完整性协议 to change keys dynamically as the system is used
• message integrity code (MIC) 信息完整性代码, prevent replay attacks
  replay attack: a valid data transmission is captured捕获 and fraudulently欺诈性地 repeated in the later session, which can be prevented by using a one-time token

Application

full scale尺度 network connection

• uses the same ISM band as Bluetooth, but with higher power (i.e., wider range) and faster connection
• requires formal configuration配置 to set up links, transmit data, and share resources.

GSM (global system for mobile communication)

General characters

• 2nd generation of digital cellular radio network 第二代数字蜂窝无线网络
• handset手机 or mobile station (MS) searching for vicinity邻近 cells to connect network 通过搜索附近的cell连接网络
• 900 or 1800MHz band, maximum transmission powers 1 or 2 watts by handsets
• cell radius : few hundred meters (town centre) up to 35 kilometers (rural area)
• data rate 9.6 and 14.4 kbps
• moderate中等 level of service security
  a pre-shared key and challenge-response
  A5/1, A5/2 and A5/3 stream ciphers

Topology

MS: mobile station
BTS: base transceiver station
BSC: base station controller
MSC: mobile switching centre

radio subsystem:

• MS: user device
  SIM (subscriber identity module)用户识别模块 holds the user specific data required for the network
• BTS contains radio equipment that enables an MS to access the network
  radio mast天线杆, antenna, transmitter, receiver
• BSC allocates frequencies to BTS’s, and implements the handover移交 procedure as an MS moves from one cell to another

network subsystem:

• MSC connects the radio subsystem to the standard fixed networks
  MSC sets up and releases the connection, handles mobility and handover requirements during the call

operation system

• operation and maintenance centre monitors and controls the network
  traffic monitoring
  status report
  security management
  accounting and billing information
• authentication centre contains algorithms for authentication and the keys for encryption加密

SIM (subscriber identity module)用户模块识别

• SIM card
• detachable可拆卸, store the subscription information and phone book securely
• identify and authenticate the subscriber on MS
• keep the information after replacing handsets
• allow users to change operators运营商 just by changing SIM 而保留手机
  SIM locking: some operators may block SIM by allowing a phone to use only one SIM or just their SIMs

Handover

• most basic form: when a phone call in progress is redirected from its current cell to a new cell
• switching from one cell frequency to a different cell frequency will be no interruption中断 since it is done electronically
• break-before-make: never release the current cell until engage the new cell. time is very short and not perceptible察觉的
  除非接上新的，旧的不会断。时间很短
• make-before-break: current source cell channel and the channel in target cell will be used in parallel for a while. so the chance is small that handover failed the call terminated终止
  两个信道一起用一段时间。因此移交失败而终止的可能性很小

Service security

• subscriber authentication: using pre-shared key and challenge-response
  MSC (mobile switching centre) prepare a 128-bit random challenge (RAND), which is sent to BTS, then to MS; MS returns a 32-bit matching signed response (SRES) to the BTS, then verified at MSC; BTS also verifies 64bits session keys received from MSC and from MS
  MSC—(random challenge)—>BTS—(random challenge)—>MS
  MS return—(signed response)—>BTS—(signed response)—>MSC verifies
  MS—(session keys)—>BTS virifies<—(session keys)—MSC
• message encryption with A5 stream ciphers
  A5/1 (used in Europe, US) is stronger than A5/2
  possible to break A5/2 in realtime
  supports multiple algorithms — operators may replace that cipher with a stronger one